1. Field of the Invention
The present invention relates generally to an image signal control system for raster graphics, and more particularly to an image signal control system provided with an outline smoothing function.
2. Description of the Related Art
In a laser printer in which an electrostatic latent image is formed by exposing and scanning a photosensitive drum or belt while switching a laser beam on and off according to image data, a smoothing operation is widely used to improve the picture quality of a printing image. The smoothing operation is performed by controlling the exposure time of the horizontal scanning direction to correct the outline of the printing image.
A conventional laser printer provided with an outline smoothing function is shown in FIG. 1. In the figure, image data received from a host computer is converted into a printing image data by a raster image processor 51, and the printing image data is stored into an image data memory 52. When the printing image data of one page is stored, the printing image data will be read out from the image memory 52 and transferred as video data VD1 to an outline correction circuit 54 by an image data output circuit 53.
The outline correction circuit 54 has incorporated therein video registers which correspond to several horizontal scanning lines (raster lines). According to the black-and-white arrangement state of surrounding dots of any dot, the outline correction circuit 54 modulates the video data VD1 such that the shape of an outline is more smoothed, and outputs the video data VD2 to a printing mechanism 55. In order to output the video data VD1 in synchronization with the scanning of the laser beam, the printing mechanism 55 supplies a horizontal-scanning synchronization signal HS and a vertical-scanning synchronization signal VS to the image data output circuit 53.
FIGS. 2A and 2B are diagrams used to explain an operation of the outline correction circuit 54. In the figure, reference character VD1 denotes an printing image data stored in the image memory 52, and reference character VD2 denotes video data modulated by the outline correction circuit 54. Assuming that a current scanning dot position is indicated by mark x. Since an outline of the video data VD1 horizontally changes to the right by one-dot step at the next raster line Li+1 of the current raster line Li, the black pattern is extended by 1/4 pitch at the previous raster line Li-1 and by 1/2 pitch at the current raster line Li, and the black pattern is contracted by 1/4 pitch at the next raster line Li+1. According to the video data VD2 outline-corrected in this way, a laser of the printing mechanism 55 is driven, an electrostatic latent image is formed on the photosensitive drum, and printing is executed by a known electrophotographic process.
However, since an image processing system of such a conventional laser printer is designed such that the outline smoothing function operates at all times, the outline correction is operated even to gradation data based on a dither method, for instance. As a result, a black printing pattern is modulated. Therefore, there was the drawback that density distribution differs from an original gradation image and the printing image quality is deteriorated.